This invention relates to the field of marine seismic data acquisition and specifically to the towing of multiple streamers of seismic receivers.
In the field of marine seismic data acquisition, streamers of receivers (usually, hydrophones) are towed through the water, receiving reflected and refracted signals from signal sources (usually, air guns). It has long been desired to increase the amount of streamers, sources, and other equipment that is towed in a survey, and various methods have been proposed. For example, European Patent Application No. 0 154 968 A2, incorporated herein by reference, discloses a method of towing one streamer with a "mother" vessel, and another streamer with a slave, submerged drone. However, such as system is difficult to control and handle, and it requires specialized equipment. Other systems have been used where multiple surface vessels tow streamers side-by-side. However, such systems are also more complicated and expensive, requiring streamers to be handled on both vessels, thus requiring a full streamer handling crew.
In practice, the most common form of towing multiple streamers is for one boat to do all of the towing. Various systems have been used for such a purpose, using various hull configurations, booms, outriggers, etc. Whatever the hull configuration, one common system, seen in FIG. 1A (where only the port side of a streamer system is shown for simplicity, the starboard side being a mirror of the port side), is to use a deflector 11 (a.k.a. "otter-board" and "paravane") to pull a deflector line 18 (in this case, a rope), laterally from the path of the vessel 14a. Streamers 10a-10d are attached to the deflector line 18. Each streamer is attached to the deflector line by a slideable mount 17 (known to those of skill in the art), which is, in turn, attached to the deflector line 18 by, for example, a rope 15a-15d.
It will be appreciated that such a system has a limit to the number of streamers 10 that can be towed at a given spacing "s" between the streamers 10, because the drag of the streamers 10 counteracts the force the deflector 11 exerts perpendicular to the path of the vessel 14a. And, as deflector angle of attack increases, to exert more force perpendicular to the path of the vessel 14a, the amount of drag also increases, resulting in reduced speed of the vessel 14a and/or a reduced amount of equipment that can be towed. Further still, it will be appreciated by those of skill in the art that control of the in-line offset of a streamer spread is important. As streamers are spaced wider and wider, the in-line offset increases beyond a desired length.
Therefore, there is a continuing, long-felt need for a marine seismic data acquisition system and method that will allow for the increase in the number of streamers that can be towed and the width of the spread, and, in general, there is a need for a system that will allow for an increase in the drag caused by the amount or distribution of equipment placed perpendicular from the path of the vessel, whether that drag is caused by additional streamers, longer streamers, or some other equipment.